Armed with state-of-the-art metal detectors, diving equipment and explosives, thieves are plundering historic sites around Britain and its coastal waters to cash in on the centenary of the First World War.

Increased interest in historic artefacts and shipwrecks, inspired by a series of anniversaries, has brought a heightened risk of theft, leading to an unprecedented crackdown from heritage and maritime bodies.

Mark Harrison, a policing and crime adviser at English Heritage, said that the value of artefacts had risen to coincide with key dates, such as the 70th anniversary of the Battle of Britain in 2010 and the centenary of